That's true but part of the issue is where are you towing. You can tow from New York to Florida over 1000 miles and never encounter any real hills. Most any rig can make the trip.
However when you turn away from the east coast and head west into the mountains it becomes more important that your combo be properly matched.
Some proclaim I been towing that way for years however they take the same 100 mile round trip over flat ground all the time.
I'm not one that believes the sky will fall if you exceed your GVW by a few pounds. But I also believe you need to weigh your rig, know your real weights and understand where your combo falls within its given towing parameters.
Assuming that its OK because you see lots of 3/4 ton trucks at the CG or you added air bags to is a mistake. Stay way from steep grades and you can get away with a few mistakes. If you travel steep grades regularly you will want your rig within its ratings.